In cities, particularly large metropolitan areas, as more and more of the land surface becomes covered with buildings or paved with streets, parking lots, and the like, a significant problem exists with respect to the disposal of the water run-off which occurs during rain storms. Parking lots and streets typically are built with slopes toward storm drain outlets which empty into underground storm sewers. These sewers usually run underneath the streets; and in many cities, the storm sewers are several feet in diameter. Even so, storm sewers frequently are overloaded and are unable to carry off the water which is dumped into them during rain storms of even moderate intensity. When storm sewers are unable to carry off the run-off water, streets, parking lots and buildings frequently become flooded, with resultant traffic tie ups and damage to buildings and the contents of such buildings wherever such flooding occurs.
Another problem which exists is that run-off water from storms is highly polluted. This water picks up oil and gasoline drippings from the roads along with other pollutants. Because of the seriousness of such pollution, such water should be processed to remove the pollutants before returning it to streams and rivers. This however, rarely is done today because of the enormous costs of providing processing plants for handling storm water drain off. Most sewage treatment plants are capable of handling only the normal sewage requirements and do not have the capacity of handling the enormously large excesses for processing storm water along with conventional waste disposal.
In many cities, particularly in the southwest, a solution to the ever increasing overloading of storm sewers has been to provide what are known as "dry-well" waste water disposal wells for localized sites where large parking lots and developments are constructed. Such dry wells typically are made by drilling a 4 ft. diameter hole in the ground to a depth of 40 to 75 ft. This hole is filled with washed gravel and a perforated sewage drain pipe, of the type typically used in drain fields for septic tanks, is inserted vertically into the center of the dry well to extend throughout the length or depth of the dry well. Water from the parking lot or other area to be controlled drains into the top of this pipe and is dispersed into the dry well. This approach to elimination of the overloading of storm sewers has proved to be quite successful. In some areas of the southwest, enough water is returned to the ground water table to stabilize the ground water level where the level prior to the construction of dry wells has been steadily dropping over a period of many years.
Problems still exist, however, with respect to dry well disposal of rain water run off. One of the most significant problems is that sand and silt which is washed from the areas being drained relatively quickly clogs the gravel around the drain holes in the drain pipe in the dry well; so that as time goes by, any given dry well is capable of handling less and less water run off. A point finally is reached when the best solution simply is to drill another dry well near the one which has become clogged. Water then is diverted to this new dry well. This is a relatively expensive solution, however, as is readily apparent.
A second problem which is also becoming of increasing significance is that the water returned to the ground water table through dry wells is polluted with oil and gasoline and other pollutants. In some sections of the country, sufficient pollution is being added to the ground water to threaten the use of such ground water subsequently for drinking water.
Unless the above mentioned problems with dry well systems are overcome, many cities are seriously considering banning further construction of run-off water systems incorporating dry wells. Accordingly, it is desirable to provide a system for use with dry wells which prevents clogging of the dry wells and which is capable of removing pollutants from the water returned to the dry well; so that the beneficial characteristics of dry wells may continue to be used without the above-noted disadvantages.